A quay crane is used as a loading-unloading machine for loading and unloading containers and the like in places such as ports. The quay crane includes travel devices which are arranged on the opposite sides with a gap in a transverse direction (also referred to as sea-land direction) being a horizontal direction orthogonal to a travel direction along the quay, a crane structure which is supported by the travel devices, and a boom which is supported by the crane structure and which extends in the transverse direction. The travel devices include a sea-side travel device arranged on the sea side and a land-side travel device arranged on the land side.
The sea-side travel device and the land-side travel device each have a travel wheel, a motor which transmits power to the travel wheel, an inverter which is connected to the motor and which controls a rotation speed (number of revolutions) of the motor, and a controller which gives a rotation speed command to the motor via the inverter.
The controllers are installed for example in an operator cabin of the crane and each give the speed command to the corresponding inverter when being operated by an operator. Each inverter supplies the corresponding motor with electric power whose frequency and voltage are adjusted based on the speed command. In other words, the sea-side travel device and the land-side travel device are controlled independently.
When a typhoon approaches, anchoring pins are inserted into through holes formed in the travel devices and the quay to fix the quay crane to the quay. To allow the through holes in the sea-side travel device to be aligned with those in the quay and to allow the through holes in the land-side travel device to be aligned with those in the quay in such event, the sea-side travel device and the land-side travel device are configured to be independently controllable.
In loading and unloading of containers with the quay crane, the operator causes the quay crane to travel in the travel direction and performs alignment such that the center of a container to be loaded or unloaded is aligned with the center of the boom. In the case of causing the quay crane to travel, the operator operates the controllers such that the sea-side travel device and the land-side travel device travel in the same direction at the same speed.
In the case of stopping the quay crane, the operator first gradually reduces the speed of each motor to 2% of a rated rotation speed of the motor which is 100% and then stops the travel devices by activating brake devices provided in the travel devices. When the speed of the motor is reduced to 0% of the rated rotation speed, that is to 0 rpm, the quay crane is sometimes pushed and moved by wind or the like. Accordingly, the brakes have been conventionally applied before the travel devices come to complete stop.
Since the quay crane has a boom protruding toward the sea, the center of gravity of the quay crane is offset toward the sea and the load (hereafter, sometimes referred to as wheel load) to be supported by the sea-side travel device is greater than the load to be supported by the land-side travel device. The applicant has found that, since the wheel load in the sea-side travel device is greater, the sea-side travel device with a relatively large wheel load falls behind the land-side travel device even when the speed commands to travel at the same speed are given from the controllers to the sea-side travel device and the land-side travel device.
When the quay crane is made to travel, the land-side travel device moves ahead of the sea-side travel device, that is, the positions of the respective travel devices are misaligned in the travel direction. The misalignment of the sea-side travel device and the land-side travel device in the travel direction generates a rotation moment about an axis extending in an up-down direction in the crane structure and strain (deformation) is generated in the crane structure. Moreover, a rotation moment in the opposite direction to the aforementioned rotation moment is generated in the crane structure as force in a direction in which the strain is released. This rotation moment causes vibration in the crane structure and the vibration causes a trouble of swinging of a boom front end in the travel direction.
Moreover, when the travel devices are stopped by applying the brakes, since the sea-side travel device and the land-side travel device are misaligned in the travel direction, the positions of the travel devices are fixed with residual strain remaining in the crane structure. Vibration occurs in the crane structure after the braking due to the effect of the strain and causes the trouble of swinging of the boom front end in the travel direction.
Booms of quay cranes include booms having a twin-box structure in which two beam-shaped members extending in the transverse direction are connected by steel members extending in the travel direction to form a frame-shaped structure and booms having a mono-box structure formed of one beam-shaped member. The booms having the mono-box structure are lighter than the booms having the twin-box structure, but have relatively low stiffness to swinging in the travel direction. Thus, the boom front end tends to swing in the mono-box structure.
Since alignment with a container to be loaded or unloaded cannot be performed in a state where the boom front end is swinging, in a conventional crane, the operator must wait until the swinging of the boom front end settles. This waiting time is necessary every time the quay crane travels and stops.
The applicant has already proposed a damping structure which suppresses swinging of a boom of a quay crane (see, for example, Patent Document 1). Patent Document 1 proposes a configuration in which damping masses are provided in a sea-side end portion of the boom and a land-side end portion of a girder to suppress swinging of the boom occurring in an earthquake. Although the damping masses can reduce the swinging of the boom which occurs in travel and stop of the quay crane, the damping masses cannot prevent the occurrence of the swinging of the boom itself. Accordingly, the waiting time is still necessary.